Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, e.g. masks (reticles) for microlithography. In this case, use is often made of masks (photomasks, reticles) which bear a specific pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern can also be provided with the aid of some other pattern unit, e.g. with the aid of a driveable pattern unit which can generate different patterns depending on the driving.
The pattern unit is positioned in a projection exposure apparatus in the beam path between an illumination system and a projection lens such that the pattern lies in the region of the object plane of the projection lens. A substrate to be exposed, for example a semiconductor wafer coated with a radiation-sensitive layer (resist, photoresist), is held in such a way that a radiation-sensitive surface of the substrate is arranged in the region of an image plane of the projection lens, the image plane being optically conjugate with respect to the object plane. During an exposure process, the pattern is illuminated with the aid of the illumination system, which shapes, from the radiation of a primary radiation source, an illumination radiation which is directed onto the pattern and which impinges on the pattern within an illumination field having a defined form and size. During an exposure process, the radiation altered by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate to be exposed, which is coated with a radiation-sensitive layer.
The illumination radiation can be characterized by specific illumination parameters for each use case. Typically, this is referred to here as an illumination setting which is specific to a use case and which can be characterized by illumination setting data.
Some illumination systems for projection exposure apparatuses can be switched for example between conventional on-axis illumination with different degrees a of coherence and off-axis illumination. The off-axis illumination settings include for example annular illumination or a polar illumination, such as, for example, a dipole illumination, a quadrupole illumination or some other multipolar illumination. The selection of the optimum illumination setting for a use case is normally made by the end user of the projection exposure apparatus in a manner dependent on the pattern to be imaged and on other boundary conditions. The illumination setting data can contain specific parameters of such illumination settings.
The illumination setting is normally defined by the user for a specific process depending on the structure of the pattern to be imaged and other influencing factors, if appropriate, and is set on the illumination system. The imaging-relevant properties of the pattern can be characterized by pattern data.
The pattern data include e.g. information about what type of structures the pattern contains and how, if appropriate, different structures of a pattern (i.e. different partial patterns) are distributed locally in the pattern. A pattern can contain for example one or a plurality of regions with lines lying very close together, which can be imaged onto the substrate satisfactorily only if the projection lens has a correspondingly high resolution capability. Those regions having the smallest line spacing and/or the smallest periodicity length (pitch) of a group of mutually parallel lines are designated as core region in this application. The lines of the core region form the “core region structure”. In order to be able to image such fine structures satisfactorily, conditions of two-beam interference are often employed by virtue of the fact that the illumination setting chosen is a dipole illumination in which the connecting line between the poles of the dipole illumination is oriented perpendicularly to the longitudinal direction of the lines to be imaged of the fine structures. Consequently, the orientation of the fine structures also plays an important part in the selection of a suitable illumination setting.
A pattern typically also contains regions with coarser structures that make less stringent desired properties of the imaging quality of the projection lens. By way of example, lines lying at a greater distance from one another can be provided, which form the feed lines to the finer structures in the finished substrate. Such structures that make less stringent desired properties of the imaging quality, in particular of the resolution capability of the projection lens, are also designated as “peripheral structures” in this application.
For the manufacturer of projection lenses the problem arises, inter alia, that it is not known what types of patterns will be used by an end user in the course of the overall use time of the projection lens for the production of structured components. This usually results in very hard specifications for the imaging quality of the projection lens in order that, if desired, even very fine structures or structures of any type can be imaged satisfactorily. The complexity and the costs for providing suitable universally useable projection lenses correspondingly increase to an extremely great extent as the requirements made of the imaging quality increase, e.g. by double patterning or quadruple patterning. The limits of the performance of the projection lenses are reached.